The most common way would be お願いがある or some mildly modified variant of it:

ちょっとお願いしていい？

The use of なにか as a direct translation is not incorrect, but it is awkward sounding and may produce confusion. In Japanese that bit is typically left out. Using the grammatical construction you were working with try instead:

やってほしいことがあるやってもらいたいことがある

There is a difference between "indefinite" and "unspecified". Using なにか sounds indefinite (even the speaker isn't sure what) whereas in English "something" is merely unspecified (the speaker knows what it is, but is not specifically mentioning what it is for the moment). こと sounds more unspecified and has the exact same functional effect as in English....seeing if the other person will cooperate or not before telling them exactly what the task is to be.